Compared with conventional energy sources, photovoltaic cells are in a difficult competitive situation from the economic point of view. The ultimate objective of photovoltaic cells, particularly for environmental reasons, is to produce current competitively with known energy sources.
Solar cells based on crystalline silicon have been successfully used in practice. Relatively large production plants are available and, in most cases, are alreay able to utilize the cost-reducing potential of mass production. However, other measures are necessary for improving competitiveness. Reducing the cost of the silicon wafers used as starting material is an important point in this connection.
A significant improvement in the cost situation is seen in the replacement of the conventional block casting process with subsequent mechanical slicing into wafers by high-speed foil casting processes.
In high-speed foil casting processes it is important to ensure that no intrinsic lattice defects are developed during the crystallization process. In general, residual impurities in the melt phase are directly incorporated in the crystallizing foil because segregation generally cannot be utilized on account of the high growth rate. Accordingly, purification of the foils requires subsequent treatment.
Numerous processes for purifying solid or molten silicon are known from the literature.
The known zone purification processes can be used for purification. In addition, it was proposed in EP-A 125 630 to subject the silicon to mechanical size reduction and to treat the size-reduced material chemically, for example by acid leaching, in such a way that the impurities are removed. However, it is not possible by this method to reduce the residual concentration of metals to the low values required of less than 0.1 ppm-wt.
U.S. Pat. No. 4,231,755 describes a process in which a roller of silicon-resistant material, of which the temperature is below the melting temperature of silicon, is rotated through a bath filled with the impurities molten silicon. In a second step, the silicon which has solidified on the surface of the roller is introduced into a zone with a temperature above its melting temperature, re-melted and collected. The residual melt enriched with impurities is periodically drained off.
Purification processes for silicon melts using reactive gas mixtures are also known from U.S. Pat. No. 4,298,423, according to which molten silicon is purified with non-oxidizing gases consisting of hydrogen, nitrogen, argon, helium, neon or mixtures thereof, preferably hydrogen. In the process according to DE-A 3 504 723, silicon in the molten state is purified by blowing in hydrogen.
According to EP-A 0 264 045, the simultaneous treatment of silicon melts with gaseous halogen compounds and steam and/or hydrogen has proved successful. The gaseous halogen compounds may be halosilanes Si.sub.n X.sub.2n+2 (n=1-4, X=halogen or hydrogen), silicon tetrachloride or hydrogen chloride. It is of advantage to add an inert gas, for example argon or helium, to the reactive gases.
EP-A 0 274 283 describes a process for the purification of size-reduced silicon under a plasma. In the first step, the silicon is melted under a plasma of 1 to 100% hydrogen and 99 to 0% argon and, in a second step, the molten silicon is treated with a plasma of argon, oxygen and hydrogen, between 0.005% and 0.05% oxygen and between 1% and 99.995% hydrogen being present.
The purification processes mentioned involve several additional cost-intensive steps and cannot be applied to already produced wafers and foils.
DE-A 3 712 443 describes the purifying effect of an oxygen treatment on silicon wafers produced by the WEB ribbon-growth process. After ribbon-growth, the silicon wafers are subjected to a separate high-temperature treatment in an oxygen-containing atmosphere. Accordingly, the process in question contains an additional cost-intensive step after the ribbon-growth process. It is not possible by this process to purify the foils during their crystallization, i.e. at the speed of the growing step.
Accordingly, the problem addressed by the present invention was to provide a process which would not have any of the disadvantages of the described processes.